1. Field of the Invention
The present invention relates to a magneto-impedance effect element having a magneto-impedance effect.
2. Description of the Related Art
FIG. 10 is a drawing of a circuit for measuring magneto-impedance characteristics of a conventional magneto-impedance effect element. FIG. 11 is a graph showing the magneto-impedance characteristics of an amorphous wire of (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 having a conventional magneto-impedance effect. FIG. 12 is a drawing of another circuit for measuring magneto-impedance characteristics of a conventional magneto-impedance effect element.
In recent years, a magnetic field sensor element having a smaller size, higher sensitivity and higher responsiveness (high-frequency operation) than a conventional flux sensing type has been demanded, and an element (magneto-impedance effect element) having the magneto-impedance effect has attracted attention with rapid development of information apparatus, measurement apparatus, control apparatus, etc.
The magneto-impedance effect means that with a small high-frequency current passed through a magnetic material having soft magnetic properties and formed in a wire, a ribbon, or the like, the application of an external magnetic field causes a sensitive change in impedance of the magnetic material. Such a magneto-impedance effect is known to be based on "the skin effect" that when an alternating current is passed through a magnetic material, the alternating current tends to flow near the skin of the magnetic material.
More specifically, the magneto-impedance effect means that for example, in the closed circuit shown in FIG. 10, when an external magnetic field Hex is applied to a wire-shaped magneto-impedance effect element Mi in the length direction thereof with an alternating current Iac in the MHz frequency band supplied to the magneto-impedance effect element Mi from an alternating current source Eac, an output voltage Emi due to the impedance inherent to the material is produced between both ends of the magneto-impedance effect element Mi even with the weak external magnetic field Hex of several Oe or less, and the amplitude changes in the range of several tens % corresponding to the strength of the external magnetic field Hex, i.e., an impedance change occurs.
An element (magneto-impedance effect element) having such a magneto-impedance effect is sensitive to the external magnetic field Hex in the length direction of the element. Therefore, for example, in use as a magnetic field sensor, unlike a conventional flux sensing type magnetic field sensor element comprising a coiled core, the magnetic field detection angle does not deteriorate even if the length of the sensor head is decreased to about 1 mm or less, thereby obtaining a weak magnetic field sensor having high resolution of about 10.sup.-5 Oe. In addition, since excitation of several MHz is possible, a high-frequency magnetic field of several hundreds MHz can be used as a carrier for amplitude modulation, and thus the cut-off frequency of the magnetic field sensor can easily be set to 10 MHz or more. Therefore, application to a new micro-magnetic head, a weak magnetic field sensor, and the like is expected.
As soft magnetic materials having the magneto-impedance effect, Fe--Co--Si--B system materials, for example, an amorphous wire of (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 (Katuo Mori, et al, "Magneto-Impedance (MI) Element" Japanese Electro-technical Committee, Reports of Magnetics study group, Vol. 1, MAG-94, No. 75-84, p27-36, 1994), etc. have been reported. As the Fe--Co--Si--B system amorphous wire, wires having diameters of 5 to 124 .mu.m are obtained. Also the Fe--Co--Si--B system materials exhibit magneto-impedance characteristics in which the output voltage Emi (mV) shows substantially symmetry in the negative and positive applied external magnetic fields Hex (Oe) with the external magnetic field Hex=0 (Oe) as a center, as shown by solid lines in FIG. 11. FIG. 11 also indicates that sensitivity rapidly increases in the range of weak applied external magnetic fields Hex of about -2 Oe to +2 Oe, causing difficulties in obtaining quantitativity in this range. Thus, the amorphous wire is unpractical as a magneto-impedance effect element for detecting weak magnetic fields.
Furthermore, since the output voltage gently changes in .the magnetic field range of absolute values of over 2 Oe, quantitativity can easily be obtained, causing practicability. However, in order to actually use a magneto-impedance effect element, it is necessary to transversely (in the axial direction of the external magnetic field Hex) shift the curve of magneto-impedance characteristics by applying a bias magnetic field Hbi of several Oe to make it easy to obtain output near the external magnetic field Hex=0 (Oe). For example, a linear portion must be placed on the axis at the external magnetic field Hex=0 (Oe), as shown by a dotted line in FIG. 11.
Conventionally as shown in FIG. 12, the magneto-impedance effect element Mi has a wire shape (or a ribbon shape), and thus a coil C is wound in an appropriate number of turns around the element Mi so that a direct current Idc is passed through the coil C to produce a bias magnetic field Hbi, to apply the magnetic field Hbi to the magneto-impedance effect element Mi in the length direction thereof.
However, the process for winding the coil C on the small wire-shaped (or ribbon-shaped) magneto-impedance effect element Mi is complicated to increase production cost, and winding the coil C on the magneto-impedance effect element Mi causes an increase in size, thereby inhibiting miniaturization in application to a magnetic head, a magneto-impedance effect element sensor such as a weak magnetic field sensor, or the like. Furthermore, since the direct current Idc is passed through the coil C to produce the bias magnetic field Hbi, electric power from a direct current source Edc is required, thereby inhibiting electric power saving of the magneto-impedance effect element sensor.